1. FIELD OF THE INVENTION
This invention relates to a single-phase three-legged core for a core type transformer, and more particularly to the structure of joints between its main leg and yokes.
2. DESCRIPTION OF THE PRIOR ART
Transportation means such as railroads and trailers are utilized for transporting power transformers to power generating stations, substations, etc. The height of these transformers is limited to a predetermined value so as not to provide a hindrance against tunnels and land bridges which may exist in the route of transportation. Such limitation is commonly called a transport restriction. A single-phase three-legged core structure 10 as shown in FIGS. 1A and 1B is advantageously employed when there is a severe transport restriction. The single-phase three-legged core structure 10 shown in FIGS. 1A and 1B comprises a main leg 1, a pair of side legs 2, and a pair of upper and lower yokes 3 magnetically coupled to the main leg 1 and side legs 2. In the core structure 10 shown in FIGS. 1A and 1B, the width of the yokes 3 is about half the width of the main leg 1, and therefore, the height of the transformer can be reduced correspondingly.
FIGS. 2A and 2B show two forms of a steel sheet 2', for example, an oriented silicon steel sheet which has a pair of diagonally cut sides C formed by cutting the end portions at an angle of 45.degree. relative to the longitudinal direction to extend between the inner side A and the outer side B. A plurality of steel sheets 1a, 1b; 2a, 2b; and 3a, 3b having shapes as shown in FIGS. 2A and 2B are laminated in layers of laminations to constitute the main leg 1, side legs 2 and yokes 3 respectively. These steel sheets are jointed in such a relation that the main leg steel sheets 1a and 1b constituting one layer of the main leg 1 are disposed in side-by-side relation to define an oil duct g therebetween, with their outer sides B confronting each other and their diagonally cut sides C arranged symmetrically. The main leg steel sheets 1a and 1b are jointed to the yoke steel sheets 3a and 3b to form joints X. The steel sheets 1a, 1b, 2a, 2b, 3a and 3b are laminated into layers of laminations while being jointed in the above manner to provide the main leg 1, side legs 2 and yokes 3.
FIGS. 3A and 3B show the sectional shapes of the main leg 1 and one of the yokes 3 when sections are taken along the lines IIIA--IIIA and IIIB--IIIB in FIG. 1A respectively. It will be seen from FIGS. 3A and 3B that the main leg 1 is substantially circular in cross-section, while the yoke 3 is rectangular in cross-section. Although the yoke 3 is shown as having the rectangular cross-sectional shape, it may have a non-circular cross-sectional shape such as a semi-elliptical or elliptical cross-sectional shape. The rectangular yokes 3 shown in FIGS. 3B are provided with oil ducts g' in communication with the oil duct g. In fact, the yoke 3 in FIG. 3B has a rear position in which the yoke 3 shown in FIG. 3B is rotatively moved by 90.degree. with the longitudinal direction of the yoke being made horizontal, however, in this FIG. 3B the yoke 3 is shown to show the relation thereof corresponding to each unit block 6 of the iron core shown in FIG. 3A. While these cross-sectional shapes of the main leg 1 and yokes 3 shown in FIGS. 3A and 3B are one of the features of the core structure for a core type transformer, these shapes give rise to a problem described hereunder.
A plurality of main leg steel sheets 1a and 1b of the same shape are laminated to constitute a core unit block 6, and a plurality of such core unit blocks 6 having stepwise varying width are stacked in tiers to constitute the main leg 1 of substantially circular cross-sectional shape as shown in FIG. 3A. In the main leg 1 thus constructed, the individual core unit blocks 6 are composed of the steel sheets 1a and 1b of different widths l.sub.11 to l.sub.18 as shown, and the width increases gradually toward the central unit block 6. Therefore, a problem as described below arises when a plurality of main leg steel sheets 1a and 1b having varying width are jointed to a plurality of yoke steel sheets 3a and 3b having the same width. Referring to FIGS. 1A and 1B again, the joints X between the main leg steel sheets 1a and 1b and the yoke steel sheets 3a and 3b have a joint angle of 45.degree.. In order that the main leg 1 of substantially circular cross-sectional shape can be jointed to the yokes 3 of non-circular cross-sectional shape at the joint angle above specified, many steel sheets 2' providing the main leg or yoke steel sheets of greater width must be clipped at both ends or one end as shown by F in FIGS. 2A and 2B so as to provide clipped ends or end D. This end clipping operation is time-consuming and will remarkably reduce the yield rate. Further, a main leg steel plate adjacent the main leg steel plate 1a forming the joint portion X shown in FIG. 1A is placed such that the main leg steel plate 1a is inverted by 180.degree. with the end D being disposed at the lower side, so that the vertices of the diagonally cut side of the main leg steel plates 1a and yoke steel plates 3a are not consistent with each other. Thus, in this case, magnetic flux does not flow through the end D, which matter causes the same problem as in FIG. 4A explained hereinbelow.
FIG. 4A shows the flow of magnetic flux .phi. at the joint X. It will be seen in FIG. 4A that the magnetic flux .phi. is relatively concentrated in the area of the inner corner of the joint X and does not substantially flow through the area of the outer corner of the joint X due to the presence of the clipped end D, and all the area of the steel sheet laminations of greater width constituting the main leg portion is not fully effectively utilized. Thus, the prior art core structure has been defective in that local losses of the magnetic flux flow lead to an undesirable increase in the core loss.
A single-phase two-legged core structure as shown in FIG. 5 has been proposed in an effort to obviate such a non-uniform magnetic flux distribution. The proposed core structure shown in FIG. 5 is quite effective in uniformalizing the magnetic flux distribution. In this core structure, however, the joint line 8 between a main leg steel sheet 1a and a yoke steel sheet 3a in one of the core layer is in the form of a straight line ST extending from an inner corner point S to an outer corner point T at an angle of .theta..sub.1, while the joint line 9 in the adjacent core layer is provided by a straight line SU extending from the same inner corner point S to another point U at an angle of .theta..sub.2. Therefore, the degree of overlap of such adjoining layers increases from the inner corner point toward the outer corner point.
Thus, in the core structure shown in FIG. 5, the lap dimension (the overlapping area of the adjoining layers) is reduced at the inner corner side, and the steel sheets 1a and 3a are butted to provide a butt joint. In the proposed core structure, therefore, errors that may occur during the steps of stacking and insertion of the steel sheets tend to give rise to formation of a gap which leads to various adverse effects and instability of the quality. This is because the magnetic reluctance increases in the area of the inner corner portion of the core, resulting in an excessive increase in the core loss and exciting current.
When the proposed joint structure is applied to a single-phase three-legged core structure, a variety of joint angles equal to the number of stepped lap joints are required resulting in time-consuming angle adjustment. In addition, due to the fact that the existence of the butt joints in the inner portion of the core increases the magnetic reluctance in that portion, the magnetic flux will be concentrated in the outer portion of the core to increase rather the core loss.
According to another prior art practice, steel sheets 2 used to constitute, for example, the yoke are cut from a coil 2' and have a pair of sides cut diagonally at angles of .theta..sub.1 and .theta..sub.2 respectively as shown in FIG. 6. The steel sheet 2 having the vertex of angle .theta..sub.2 at the front end or in the advancing direction X is clipped as shown by E to provide a clipped end D. This manner of end clipping is only applicable to alternate ones of the steel sheets 2, that is, those having the vertex of angle .theta..sub.2 in the advancing direction. This is because the steel sheet 2 having the vertex of angle .theta..sub.2 ' in the direction opposite to the advancing direction X is difficult to be engaged by a stopper 15, whereas that having the vertex of angle .theta..sub.2 in the advancing direction X can be easily engaged by the stopper 15. In order to clip the former steel sheets 2, another cutter must be separately provided resulting in a complex cutting step.